i) Field of the Invention
This invention relates to a preformed sheet membrane, which can be used in the construction of buildings to control the movement of air and water through the building envelope. The invention also relates to an assembly comprising a wall structure which incorporates the membrane, and a method of assembling such a wall structure.
ii) Description of Prior Art
Moisture problems in walls have been attributed to two principal mechanisms: water vapor diffusion and air leakage. In recent years, control of air movement has become recognized as a major factor determining building performance, as measured by how well a building functions during its lifespan. Depending on local climate, air leakage through walls can result in excessive efflorescence, spalling of masonry, frozen pipes, condensation and ice buildup in cavities, wet and dysfunctional insulation, mold growth as well as rain penetration, high energy costs and poor control of the building environment.
Air leakage is the uncontrolled movement of air through the building envelope. This movement of air into a building (infiltration) and out of a building (exfiltration) is caused by pressure differences produced by wind, stack or chimney effect and fan pressurization. Air leakage may follow such paths as holes or openings through the envelope, for example, cracks or joints between infill components and structural elements or through porous materials such as concrete block and porous insulation materials.
Older methods of building design often relied solely on the use of vapor barriers or retarders, such as polyethylene film, to control water vapor movement in the building envelope. The vapor barrier retards the diffusion of water through the assembly of materials in a wall. The rate at which water vapor migrates or diffuses through a material depends on two factors: the difference between the water vapor pressure in the air inside the building and that in the outside air, and the resistance that materials present to the migration of water by diffusion. A vapor barrier is a material that offers a higher resistance to the diffusion of water vapor than most other materials. Polyethylene film of sufficient thickness is the material most commonly used for this purpose; however, other materials such as aluminum foil, some paint products, some insulation mastic adhesives and some mastic coatings have been used as vapor barriers. The moisture diffusion control property of a material is its water vapor permeance. This is usually expressed as the weight of moisture that will diffuse through a given area over a specified period of time at a unit vapor pressure difference. According to Canadian Standard CAN/CGSB-51.34-M86, a polyethylene sheet vapor barrier must have a maximum water vapor permeance of 15 ng/Pa·s·m2 when tested in accordance with ASTM E 96.
For the vapor barrier to control condensation resulting from vapor diffusion, it must be placed on or near the warm side of the insulation, which is normally the high vapor pressure side.
Water vapor diffusion is one of the mechanisms by which water can be transported into a wall or roof cavity. The provision of a vapor barrier within the wall or roof assembly satisfies only part of the requirement of controlling moisture entry into building enclosures. The other mechanism, which is now considered to be far more significant, is air leakage. Both mechanisms may, however, operate at the same time.
The principal function of the air barrier is to stop outside air from entering the building through the walls, windows or roof, and inside air from exfiltrating through the building envelope to the outside. This applies whether the air is humid or dry, since air leakage can result in problems other than the deposition of moisture in cavities. Exfiltrating air carries away heating and cooling energy, while incoming air may bring in pollution as well as disable a rain screen wall system.
Moisture-laden air passing through an insulated cavity with a vapor barrier may deposit much more moisture than would diffuse through the vapor barrier in the same period of time. Recent practices are increasingly recognizing the importance of incorporating an air barrier system in building designs.
Materials and the method of assembly chosen to build an air barrier system must meet several requirements if they are to perform the air leakage control function successfully.                1. There must be continuity throughout the building envelope. The air barrier material of the wall must be continuous with the air barrier material of the roof (e.g., the roofing membrane). The air barrier material of the wall must be connected to the air barrier material of the window frame, etc.        2. The air barrier system must be fastened to a supporting structure to resist a peak wind load, a sustained stack effect or pressurization from ventilation equipment; it must be sufficiently rigid to resist deplacement.        3. The air barrier system must be virtually air-impermeable. According to the requirements of Part 5 of the 1995 National Building Code of Canada, the material in the air barrier system intended to provide the principal resistance to air leakage is required to have an air leakage characteristic not greater than 0.02 L/s.m2 measured at a pressure differential of 75 Pa.        4. The air barrier assembly must be durable in the same sense that the building is durable, and be made of materials that are known to have a long service life or be positioned so that it may be serviced from time to time.        
A wall or roof assembly will require an air barrier and possibly also a vapor barrier. They may or may not be the same material. But a combined system must meet the design requirements for both functions.
A vapor barrier is usually placed on the warm side of the insulation. It may also be positioned part way into the insulation but, for satisfactory performance, it should be no further in than the point at which the temperature of the inside air drops to its dew point. While it is preferable that the air barrier system be placed on the warm side of an insulated assembly, it is not an essential requirement as it is with the vapor barrier. The position of the air barrier in a wall or roof is more a matter of suitable construction practice and the type of materials to be used. However, if this barrier is positioned on the outside of the insulation, consideration must be given to its water vapor permeability in case it should also act as a barrier to vapor which is on its way out from inside the wall assembly. This situation may be prevented by choosing an air barrier material that is ten to twenty times or more permeable to water vapor diffusion than the vapor barrier material, in order to prevent high humidity conditions and the potential of mold development or condensation in the wall assembly. It is this situation that demonstrates the need for a water vapor permeable (or breathable) air barrier membrane.
According to Canadian Standard CAN 2-51.32-M77, a breather type sheathing membrane must have a water vapor permeance of no less than 170 ng/Pa·s·m2 when tested in accordance with ASTM E 96.
In the current state of the art, there are products available both in preformed sheet form supplied in rolls and in liquid form to be applied by bush, spray or trowel that can serve as air and moisture barrier membrane materials. As examples of preformed sheet products, there may be mentioned Blueskin (trade-mark) SA Self-Adhesive Air & Vapor Barrier Membrane and Blueskin (trade-mark) TG Thermofusible Grade Air & Vapor Barrier Membrane as manufactured by Bakor Inc. These products are based on a polymer modified bitumen laminated to a polyethylene film surface and, in the case of the Blueskin TG, reinforced with a non-woven fiberglass. Canadian Patent 1,261,239 describes an air barrier membrane, consisting of a reinforcing sheet of organic fibers coated on both sides with a bitumen binder, which can be applied using a torch or which can be self-adhering and cold applied by pressure. Bituminous materials and polyethylene films are known to be excellent vapor barriers.
Materials similar to the above are also used as waterproofing membranes, which are normally installed on foundation walls below ground level, as opposed to the air and moisture barriers discussed herein, which are normally installed on exterior walls above ground, and roofs, to protect against the infiltration and exfiltration of air and the infiltration of wind driven rain. Some such materials are described in Canadian Patents 861,467 and 935,371.
As examples of liquid applied air and moisture barrier products, there may be mentioned Air-Bloc 06 (trade-mark) Elastomeric Liquid Air & Vapor Barrier and Air-Bloc 31 (trade-mark) Liquid Emulsion Vapor Permeable Air Barrier Membrane, as manufactured by Bakor Inc. Although the technology exists for a vapor permeable air and moisture barrier membrane of the liquid applied type, there does not currently exist a preformed sheet membrane which entirely meets the practical requirements for providing an air and moisture barrier system as previously outlined. Liquid applied membranes themselves need to have preformed sheets incorporated into the air and moisture barrier system to tie the liquid applied membrane into beams, window and door frames and to connect the roof membrane in order to provide continuity of the system. Sheet membranes are often preferred over liquid applied membranes because sheet membranes come with factory-controlled thickness.
Some preformed sheet membrane products offer a partial solution to providing a water vapor permeable or breathable air barrier system. One such product is Tyvek− (trade-mark) spunbonded polyolefin sheet as manufactured and sold by DuPont under U.S. Pat. No. 3,532,589. This type of product presents a number of difficulties in achieving an air barrier system in that multiple accessory products are required in order to create a continuous plane of air tightness throughout the building envelope. These accessories include mechanical fasteners, nails or screws, with large diameter metal or plastic plates or washers to secure the membrane, seaming tape to tape the joints of the membrane and flexible flashing for all transition areas such as tie-ins to window frames. The use of a tape, and multiple mechanical fasteners which puncture the membrane compromise the integrity of the system.
In addition, further difficulties occur when attempting to mechanically fasten this type of product on commercial buildings over such substrates as concrete block as opposed to wood frame housing. Further, because this type of membrane is not fully adhered to the substrate, it allows air to move freely behind it, to find an opening or defect in the membrane through which it can escape.
A potential solution to the deficiencies exhibited by current breathable sheet materials would appear to be in the use of an adhesive, applied to one side of the sheet, to enable the sheet to be self-adhered to the substrates, thus eliminating the need for mechanical fasteners and tapes. However, adhesive films as normally applied tend to substantially reduce the water vapor permeance of the sheets.